Operation with NULL value                   
We have not introduced the NULL value up to this point. Let’s take the example of the numeric object type in order
to show how to work with the NULL value.

One of the possibilities is to use the object caption in order to start working with the NULL value, for example ‘double click’.

After the action of moving to work on NULL value, we get he image of NULL object.

This directs our attention to the following:
             1)       If we repeat the action (double click mouse), we move back to the previous status.   
             2)       Filter string value is for the object which is temporarily visible

In the table below, there is a list of objects which can form the NULL value.